OBJECTIVES: To develop a user-oriented transcutaneous ultrasonic Doppler bloodflow measurement system featuring an easy-to-calibrate absolute accuracy. To realize this the following quantities will be monitored on line: a. the angle of ultrasonic illuminations, b. the directional absolute flow velocity spectrum, c. the directional average flow velocity, using a rational calculation procedure, d. the artery diameter to produce a calibrated volume flow. APPROACH: A novel probe will be constructed, permitting the full spatial information of the vessel to be measured, and producing a homogeneous parallel-beamed whole flow illumination to give the resulting Doppler spectrum an unambiguous relation to the whole bloodflow. Progress will be monitored, and the limits of absolute accuracy will be established, by in vivo calibration measurements on chronically cannulated carotid arteries of sheep. The practical value will be tested through diagnostic use in our Large Animal Clinics.